I. Field of the Invention
This invention relates to insulating lightweight refractory materials and to compositions used to form such materials. More particularly, the invention relates to insulating lightweight refractory materials which are resistant to attack by molten metals, especially to molten aluminum and its alloys.
II. Description of the Prior Art
During the production of metals such as aluminum, zinc, lead, copper, tin and their alloys, and during the casting of shaped articles from such metals, various implements and structures come into direct contact with the molten metals and must be able to resist the consequent high temperatures and chemical attack. The implements and structures should also preferably be fairly good heat insulators in order to avoid rapid cooling of the molten metals. In order to provide the necessary properties, the implements and structures are often made out of ceramic materials or have protective ceramic coatings. Ceramic-type refractories have good resistance to high temperature but may be susceptible to thermal stress cracking or to chemical attack by molten metals. For example, molten aluminum is fairly reactive with silicate-containing refractory materials, and certain aluminum alloys (notably 4.5% Mg/Al and 2.5% Li/Al) are extremely chemically aggressive. These metals tend to form destructive Al-Si-Al.sub.2 O.sub.3 cermets from silicate refractories.
In order to counteract this destructive process, the usual practice has been to use various washes and coatings to protect the exposed refractory surfaces. Examples of such washes and coatings are mica wash, wollastonite, boneash, zirconia, thoria, titania and, recently, various carbides and nitrides. These however adhere only slightly to the surface to be protected and eventually find their way into the processed metal as undesirable impurities.
There is consequently a need for a highly insulating, light refractory material having improved resistance to molten aluminum and other related molten metals, and for a composition used to form such materials.